This invention relates to current transfer brushes in general and more particular to a current transfer brush with a multiplicity of fibers which are combined to form a slider member and extend at least approximately perpendicular to the contact surface of the brush, the brush having parts of electrically conductive material.
The brushes in electric machines are used for transfering current between a stationary machine part and a rotating machine part. By using graphite, good electric conductivity and, at the same time, good sliding properties on the contact member connected to the rotating machine part, for instance, a slip ring or a commutator, are ensured. The running properties of such a brush are determined mainly by the friction coefficient .mu. as a function of the circumferential velocity of the contact member connected to the rotating machine member and by the voltage drop .DELTA. U as a function of the current density transferred via the brush. Both quantities depend heavily on the extraneous skin formed on the rotating contact member, which is also called film or patina. This extraneous skin is composed of the materials of the slider member of the brush and the contact member abraded during operation. Its thickness and nature is influenced by a multiplicity of factors. Thus, it is determined, for instance, by the material composition of the graphite in the brush and of the contact member, by the current density provided, as well as by the circumferential velocity and the temperature of the contact member. In addition, it depends on the contact pressure of the brush and also on the ever changing influences of the atmosphere and altitude climate, the humidity of the air and chemically agressive gases and vapors.
The slider members of such graphite brushes can contain a multiplicity of fibers of carbon or graphite which are combined in a bundle and are coated with a metal film of high electric conductivity (British Pat. No. 1,191,234). In this case graphite fibers in the form of a rope with several thousand individual fibers are used as the starting material. Such fibers are known, for instance, from the British patent No. 1,110,791. The advantages of these fiber brushes over the known brushes with an electro graphite block are that there are substantially more contact points between the slider member and the rotating contact surface, the fibers are highly elastic, and the electrical data and the mechanical running properties of the brush are improved thereby.
Metallizing the individual graphite fibers, however, is relatively expensive. With the known methods for electroless or electroplating deposition, furthermore, only a limited choice of metals can be applied to the graphite fibers. With these chemical methods, there is, in general, the difficulty that the electrically conductive material to be applied does not adhere well to the graphite material. Therefore, precoating with a substrate material which adheres better is often necessary, on which, in a further process step, the desired electrically conductive material can then be deposited. With this method, there is also the danger that individual fibers will stick together during the coating process and thus, nonuniform coating is obtained. Because of these difficulties, physical methods are preferred. Especially advantageous is metallizing the fibers by ion plating. Ion plating is understood to be a an evaporation process, in which the atoms or molecules to be deposited are partially ionized in a plasma and impinge on the graphite member to be coated in an electric field with higher energy ("Vakuumtechnik," 1976, pages 65 to 72 and 113 to 120). In this process, the substrate, which is necessary, for instance, with electro deposition methods, can be dispensed with. In addition, a relatively uniform coating of all fibers of a fiber bundle is possible without the need for additional measures. Suitable apparatus for the coating of the graphite fibers of a fiber bundle is relatively expensive, however.
With the known coating methods, moreover, only relatively small layer thickness of electrically conductive material can be applied to the individual fibers. It is therefore possible to adapt a brush composed of such fibers to the pecularities of different types of machines only to a limited extent. Thus, for instance, the ratio of metal to graphite cannot be set arbitrarily without additional special measures.